She married him because he was known as the most honest and trustable man among the people that time and Khadija (the first wife 40 yrs old and our prophet was 25) wanted an honest man to look after her business, and no one was known to be better than our prophet Mohammad, and the God words (Wahy) came to him during his marriage to her, the Wahy came by the angel who is called Gabriel.
In Islam the woman has the right to have her own property and business and also if the man is not capable of doing his responsibility but the women can then the leadership will go to the women who could be the wife or the sister or ...etc (relatives).